Communication networks whose components communicate with one another on a packet-switched basis are replacing circuit-switched communication networks to an increased extent. Such directly communicating networks are frequently also called Voice-Over-IP (VoIP) networks if the Internet protocol (IP protocol) is used for interchanging the audio information which is present in the form of data (voice data). In this case, the networks may be used either exclusively for transmitting voice data or else can transmit both voice data and other information (e.g. data, video) as a mixture.
In circuit-switched communication networks, terminals (e.g. telephones, fax machines) are always connected to a communication node (e.g. a telephone exchange). In this case, any flow of information to and from the terminals involves the respective communication node, with control information for setting up connections, for example, being interchanged between the terminal and the communication node while, although the useful information (the analog voice signal or the modulated fax signal) is interchanged between the terminals involved in the communication link, it is forwarded (transmitted) by at least one communication node involved in the communication link.
In voice data networks, the terminals are frequently called “clients”, because the terminals in these communication networks are operated in a similar manner to a computer in a data network and may be in the form not only of a telephone or a fax machine, but also of a multimedia PC, for example. The latter terminals are computers which have suitable hardware for inputting information (e.g. a microphone, a camera, a keyboard, a scanner etc.) and outputting information (e.g. a screen/display, loudspeakers, LEDs etc.). Clients may also be in the form of a miniature PC (a “PDA”—Personal Digital Assistant) or in the form of an added-feature mobile telephone (“smart phone”). Said appliances are often equipped with a wireless data link and are then, in the case of mobile telephones, also called “WLAN mobile phones” (WLAN=Wireless Local Area Network).
The clients in the voice data networks usually interchange the useful information with one another directly. To this end, after a communication link has been set up, two clients communicating with one another respectively know the network address of their communication partner, and they use these network addresses to interchange the data packets containing the useful information with one another. To set up a connection, on the other hand, central network nodes (communication nodes) are also used in most known voice data networks. Such network nodes are known as “gatekeepers” to the voice data networks using the H.323 protocol (ITU-T-H.323) and as SIP proxy servers in the voice data networks using the SIP protocol (IETF-SIP; SIP=Session Initiation Protocol).
In widely ramified data networks, particularly on the Internet, communication structures called “Peer-to-Peer” networks or else “End-To-End” networks are increasingly being used. A primary area of application for such communication structures is the interchange of files, known as “filesharing”. For this, a subscriber (client) in such a communication structure interrogates a number of other clients (typically computers) on the search for a particular resource (which may be a music file or a computer program) and, when this resource has been found, starts to use this resource, which usually involves the transmission (“download”) of a file or a plurality of files. In this case, appropriate communication partners (clients) can either be found using a database, or else the clients can be found by appropriate communication methods automatically, i.e. without a central entity.
Some of the known “Peer-To-Peer” networks are used for the purposes of direct communication. The “media data”, that is to say the useful data, interchanged in the process are not limited to voice (voice data) or modulated information (e.g. fax transmissions), but rather also include moving-picture information (video transmission, video conferencing, “multimedia messaging”) and other forms of electronic communication too. In such communication networks, which are also called “directly communicating communication networks”, it is thus firstly possible to implement communication links in the style of conventional “telephone calls”, and it is secondly possible to transmit expanded contents (multimedia) too. In this case, dispensing with central network nodes (communication nodes) has the advantage of increased failsafety, which has the associated drawback that the (switching) functionality of the network nodes known from circuit-switched telephony needs to be provided by the clients involved in the communication link themselves.